1. Field of the Invention
This invention relates to an auxiliary air system for a gas turbine engine which affords new and improved means for measuring the engine inlet air temperature and for cooling engine components.
2. Description of the Prior Art
The sophistication of gas turbine engines has developed to the utilization of electronic engine controls to supplement, and in some cases replace, hydromechanical engine controls for providing improved safety and higher efficiency of operation. However, an electronic control module is more sensitive to temperature than is a hydromechanical control and therefore requires a greater degree of cooling to maintain its reliability in normal operation and to prolong its useful service life. One manner of keeping the module cool in a gas turbine engine, such as a turbojet engine, is to install it in the nacelle between the engine core, that is, the center of the engine comprising the compressor, combustor and turbine, and the engine cowling, that is, the external, metallic covering of the engine, where the temperature is lower than it is within the engine core. On a turbofan engine, the module can be installed in the annular airspace within the nacelle between the fan casing and the cowling where, because this portion of the nacelle is spaced away from the engine core, the temperature is lower than it is immediately adjacent the core. However, the temperature in the nacelle of any gas turbine engine is still too warm to enable best, long-life operation unless supplemental cooling is utilized. Such supplemental cooling can be obtained by blowing air across the module, and is most advantageous if the coolest air available is utilized to obtain the greatest amount of cooling. The source of air commonly used for cooling engine components such as an electronics module is air bled from the initial stages of the compressor of a gas turbine engine, or, in a turbofan engine, fan air from behind the fan. The air from each of these sources has pressure increased by the compressor or fan, and is thus warmer and consequently a less desirable source of cooling air than is the unpressurized outside air surrounding the engine nacelle. Further, utilization of air from such sources is detrimental to the overall powerplant efficiency. Thus, the most efficient cooling medium is the outside air that enters or passes by the nacelle. Further, the ultimate purpose is to cool the electronic components within the module, not to cool the total environment around the module. Thus, cooling systems which simply pass air around the outside of the module are not the most effective means to achieve maximum life and reliability of the electronic components.
Other engine accessories must also be cooled to operate properly. Further, the compartment which contains the accessory gearbox and its associated components and lines for handling flammable fluids must be properly ventilated to protect against the accumulation of combustible vapors. As indicated above, compressed air from a source within the engine has a higher temperature than the air surrounding the nacelle and is therefore not as efficient for cooling as is the outside air. Additionally, if ram air, that is, the air which is forced into the engine as the aircraft moves through the air, is utilized for cooling and ventilating, there is no flow of air when the aircraft is stationary on the ground.
Another important function in the efficient operation of a gas turbine involves the determination of engine inlet air temperature. The thrust developed by a gas turbine engine and the engine control settings are dependent in part upon the temperature of the air entering the engine. Consequently, this inlet air temperature must be measured to adjust the fuel flow into the engine for obtaining the desired thrust. Inlet air temperature sensors are commonly located at positions on the engine nacelle upstream of the engine compressor, and upstream of the fan in the case of a turbofan engine, such that the sensors are directly exposed to the engine inlet airstream. However, this placement can result in inaccurate readings or even loss of temperature measuring capability. For example, the sensors may accumulate an ice coating at some atmospheric conditions, or they may experience foreign object damage from bird strikes or earth particles impinging upon the sensors. An even more serious difficulty may arise if the sensors are located on the inner surface of the engine inlet cowl. If a sensor, or part of one breaks off, as could occur, for example, during a bird strike, the loose piece would be ingested by the fan or compressor and could cause serious damage or even lead to engine failure.
In view of the cooling problem discussed above, a primary object of the present invention is therefore to utilize the cooling efficiency of air from outside the engine nacelle for cooling engine components located within the nacelle.
Another object of the present invention is to provide a new and improved means for measuring inlet air temperature which removes the sensors from ahead of the engine inlet and places them in a protected air passage within the engine cowling.